Wetting behavior of a liquid on a substrate surface is typically a function of the surface energy of the substrate surface and the surface tension of the liquid. At the liquid-substrate surface interface, if the molecules of the liquid have a stronger attraction to the molecules of the substrate surface than to each other (the adhesive forces are stronger than the cohesive forces), then wetting of the substrate surface generally occurs. Alternatively, if the molecules of the liquid are more strongly attracted to each other than to the molecules of the substrate surface (the cohesive forces are stronger than the adhesive forces), then the liquid generally beads-up and does not wet the surface of the substrate.
One way to quantify surface wetting characteristics of a liquid on a surface of a substrate is to measure the contact angle of a drop of liquid placed on that surface. The contact angle is the angle formed by the solid/liquid interface and the liquid/vapor interface measured from the side of the liquid. Liquids typically wet surfaces when their contact angle is less than 90 degrees. Typically, a decrease in the contact angle between the liquid and the surface correlates with an increase in wetting. A zero contact angle generally corresponds to spontaneous spreading of the liquid on the surface of the substrate.
For many applications (e.g., sensors and microfluidic devices), the ability to precisely control the wetting and/or flow of a liquid on a surface of a substrate according to a precise high-resolution pattern can be important. Thus, it would be desirable to have additional methods and materials that can provide such control.